It must first be emphasized that production well-logging may play an essential role in the operational strategy of working a horizontal or sharply slanting oil well, if well-logging can be performed correctly. It is generally agreed that one horizontal well may replace several vertical wells (generally two to four) both from the standpoint of the output they may furnish (increased production index) and that of recovery (increasing the drainage area and decreasing water coning problems).
Although this dual advantage of the horizontal well is valid in the case of a homogeneous reservoir, this cannot be the case in the far more frequent instance of heterogeneous reservoirs. Because of the presence of heterogeneities, the total output of the well may not be profitable because of inflowing water, which may be characterized by an overly high "water-cut ratio" (Quantity of Water divided by Quantity of Liquid) or an overly high Gas Oil Ratio (GOR). The output may be too low, for example, to confine the GOR to an acceptable value, even though this production problem may arise from a limited portion of the drain. Even if this type of problem does not lead to automatically excluding the use of horizontal wells for this type of deposit, it is clear that the horizontal well does not in this case offer the flexibility the producer may desire to optimize exploitation of the field. Moreover, it should be noted that the set of vertical wells that may replace the horizontal well would offer more opportunities, as the vertical well draining the part of the reservoir responsible for the production problem may be easily shut off without harming production from the other wells.
This problem may obviously be avoided by using selective completion in the horizontal drain, allowing either production to be modulated section by section, or the problematic section of the drain to be shut off.
The use of selective completion may be conceived as two different stages in the life of a well: either immediately after the well has been drilled, or later, at the time the need for its use arises.
In the first case, it is clear that the decision to use selective completion is a difficult one for several reasons, namely, the additional cost involved in the selective completion equipment must be justified at the outset, and then the sections to be individualized must be defined from a static description of the reservoir.
The postponed decision has the advantage of being taken once information is available. The additional investment will be applied only to the wells that require it, and only at the time it becomes necessary. In most cases, it will be made only after the payback period of the well. Moreover, it may be easier to define which sections are to be isolated if dynamic data are also available on the reservoir, particularly by using production well-logging.
On the other hand, this intervention may be made difficult, if not impossible, by the temporary completion used during the first operating phase of the well, for example by using a non-cemented perforated liner (generally called preperforated liner by the specialists).
In addition, this production method (first phase non-selective, second phase selective) may, in certain cases, reduce eventual recovery.
The first solution (selectivity from the very start of production) thus appears to be more attractive technically, but not necessarily economically. The solution of cementing and perforating a liner over the entire length of the drain, which allows for selectivity thereafter, must in certain cases be abandoned for cost reasons.
Therefore, the best solution is to accomplish the first production phase in an open hole; however, this is not always possible because of uncertainties as to the mechanical integrity of the well.
As a result, the case most frequently encountered is that of non-cemented wells.
Whatever the type of completion used for the horizontal well, when a problem of undesirable fluid production arises, it becomes important to locate the section or sections that might be responsible for this production, and to evaluate the well's potential when these sections have been shut off.
Only production well-logging can provide the necessary answers. However, its implementation comes up against difficulties relating both to the horizontal design and to the method of completion.
Of all the possible methods of selective completion (total cementing or partial cementing, formation packers) or non-selective completion (open-hole, preperforated liner), the case of the perforated liner is the one that brings together all the difficulties. This is the case that will be considered hereinbelow, as production methods with other types of completion may be arrived at by introducing the appropriate simplifications.
The problems inherent in production measurements in horizontal wells arise from a combination of difficulties of interpretation that are known from vertical wells, and difficulties inherent in horizontal wells principally due to the way in which the sondes are moved, the particular effect of gravity, and the type of completion peculiar to this type of well (large liner diameter, liner often not cemented, etc.).